Safety control for gaseous fuel burners



Sept. 3, 1946. s, G. E$K|N ET AL v 2,406,752

SAFETY CONTROL .FOR GASEOUS FUEL BURNERS Filed April l0, 1943 Patented Sept. 3, 1.946

SAFETY CONTROL FOR GASEOUS BURNERS FUEL Samuel G. Eskin and Charles K. Strobel, Pittsburgh, Pa., assignors to Robertshaw Thermostat Company, 'Youngwood Pa., a corporationof Pennsylvania f' Application April 1o, 1943, serial No. 482,592

claims. `(c1. 15s-117.1)

Thisiinvention relates to control systems and, more particularly, tocombined safety and condition controlling means for fuel burners and the like In application S. N. 439,148 filed April 15, 1942,

now Patent No. 2,366,774, granted January 9, 1945, there is disclosed aV safetyfsystem for fuel burners employing both av flame or heat sensitive y. device anda current sensitive device for controlling operation l of Van' electromagnetic fuel valve. The valve in this systemV was used solely for safety purposes and would function to terminate the supply of fuel to the main burner upon extinguishment offthe flame to which the flame sensitive device was exposed. In this present invention it is proposed to employ the electromagnetic valveas disclosed inthe former application, both to regulate the ow of fuel to the main burner for temperature regulation and also terminate suchflow in the event of. ame

failure. It will readily be apparent that in its broad aspects the present invention may be utilized both for condition and safety control as previously indicated.

An object of this invention is to permit utilizav and condition controlling apparatus for a fuel f burner embodying the invention;

Fig. 2 is a schematic View of a detail in one operating position; and l Fig. 3 is a schematic View of the same detail in another operating position.

Referring more particularly to the drawing, a gaseous fuel burner I0 and an associated pilot burner I2 are shown as being supplied with fuel by a main fuel pipe I4. The fuel supply to both burners is controlled by. a main valve I6 which may be manuallyoperated. Interposed in the fuel supply line I4 between the main valve I6 and the burner I0 is an electromagnetic or solenoid gas valve VYI8 of any known type which maybe arranged, asin this instance, to be opened when Lminimum value. trol-valve I6 from'closed to open position will sufficient energizing current is supplied to the solenoid of valve I8 and to be closed by gravity when such current is reduced below a certain Operation of the main conpermit ow of fuel from the pipe I4 to the pilot burner I2, but fuel is not supplied to theV main burner IIl'unless the solenoid Valve IS is also in open position. K,

, Electric current for the operation of the appa'ratus is supplied by the line wires 20 connected to the primary of a transformer 22 which, in this instance, receives commercial current at the primary where a voltage variation from -to volts -can occur.l In this embodiment the main valve I6 has a main switch combined therewith which is connected to one line wire 2Uv by wires 24 an'd- 26. Any suitable means may be incorporated in the combinedY valve and switch structure to close the switch when the main valve Itis-actuated to open position. The secondary of the transformer 22 delivers the current from line wires 20 at voltages varying in accordance with the primary Yvoltage variation above and below an averager of six volts. Connected to one terminal of thetransformer secondary by a wire 28 is one junction A of a parallel mesh of which a name sensitive element 30 forms one branch and is connected at'oneend by wire 29 to wire 28 and at the other end by wire 3| to wire 32 at the opposite junction B of the mesh. .The element 30 is positioned adjacent the pilot burner I2 exposed to the heat of the flame thereof and may be in 1 the form of a coil of wire formed from material having normally a relatively low resistance and a positive temperature coeflicient of resistance forming a ballast resistor. It will be apparent that other types of flame sensitive, heat responsive or thermal resistance elements havingV the required characteristics may be substituted and that it is not essential that the flame of the pilot burner I2 actually impinge on the coil.

Therother terminal of the flame sensitive element 30 is connected by wires 3l and 32 with one end of another parallel mesh included in the secondary. circuit, the other end of this parallel mesh being connected by a wire 34 to the other terminal of the secondary of the transformer 22. One branch of this parallel mesh comprises a resistance` element 36 whichy will be hereinafter designated as a valve control resistor. This resistor 36 is preferably in the form of a coil of wire of material having normally a relatively low resistance and a positive temperature coefficient of resistance forming a ballast resistor, althougnas will appear hereinafter, a constant resistor could be employed. The resistor 35 is not, however, eX- posed to the heat of any flame but is adapted to become heated upon passage of electric current therethrough and will change its resistance in accordance with changes of temperature caused by changes in the value of current fiowing through it.

The other branch of the parallel mesh comprises a constant resistance elementA 53 forming a pick-up resistor for the solenoid valve, and' a control switch connected in series circuit therewith. The control switch may take the form of a thermostat composed of a bimetal arm lll! carrying on one end thereof a pair of contacts HI, 43 disposed one on each side and which upon warping of the arm will engage alternately with contacts l2 and (it, designated hot contact and cold Contact respectively, and spaced one from the other. The control switch may be placed in the space tobe heated by the main burner It and the arm t3 be subjected tothe temperature variations therein forming a space thermostat of special form. To this end, the cold contact t3 is preferably carried upon a second bimetal arm M which is adapted to warp upon passage of electric current therethrough so that only the hot contact ft2 can be considered as a relatively iixed contact for the thermostatic switch. A limit stop 48' is interposed between the bimetal arms 46 and 44 in position to be engaged by thearm d to restrain movement thereof toward the arm i4 for a purpose which will be apparent hereinafter.

The hot contact 42 is connected by a wire 53 to one end of a constant resistance element 52, the other end of which is connected by a wire 5I to wire 23 at junction A' of the parallel mesh. Another branch of this mesh includes a cut-out 58 which is thermally sensitive and may be of the slow acting bimetallic snap switch type. One terminal of the cut-out 58'is connectedby a wire 53 to the wire 23 at junction A and the other terminal is connected by'wire 54 to one terminal of the solenoid valve I8. The other vterminal of the solenoid valve I3 is connected by a wire 56 to the wire 32 at the junction B, thus placing the' name sensitive element' 33 in parallel circuit with the cut-out 58 and solenoid valve i3.

InV the operation of the apparatus the closing of the main switch of' valveV I6 is effective upon manual opening of this valve, as described, and permits fuel to flow to the pilot burner I2.` As the solenoid valve I8 remains closed, no fuel can flow to the main burner I at this time. The circuit thus-established may be traced from the line wire 23 to wire 23, electric switch of main valve I6 and wire 26 to the primary of the transformer 22, from the secondary of the transformer by wire 28 to wire 53, cut-out 58, wire 54, to one terminal of solenoid valve I8, wire 56 to wire 32, valve control resistor 33 and wire 34 to the other side of the transformer secondary. Current also flows in the shunt established by the flame sensitive element 3l! which is connected by wires 29 and 3I around the coil of the solenoid valve I3 causing the element 30 to become heated to some extent. During this starting operation the control thermostat is usually in cold-contact position so that pick-up resistor 38 is connected in parallel circuit with the valve control resistor 33. The current values in the circuit are adjusted so that when these circuit conditions exist and the flame sensitive element 3i! remains unheated by the pilot burner flame, there is insuflicient current flowing to the coil of the solenoid valve i8 to actuate this valve to open position.

The fuel issuing from the pilot burner l2 may be ignited by a match and the relatively small heating effect of the current on the flame sensitive element 3i? is amplified by themuch greater heating effect of the pilot burner which will then occur. Assuming that the control thermostat is in the cold contact position with the contact 46 engaged by related contact on arm All), then the circuit Will be as previously described, that is, the parallel mesh of pick-up resistor 35i and valve control resistor 36, the parallel mesh of solenoid valve I3 with its cut-out 58 and name sensitive element 3l), all in series with the transformer secondary. Upon heating of the flame sensitive resistor 30 by the pilot flame in this starting or cold Contact position, the shunting of the valve control resistor 36 by the pick-up resistor 38 and cold contact 423 causes sufficient currentV to ow through the solenoid valve I8 to actuate this valve to open position. As in the case of application Serial No. 439,148 previously referred to, this effect is produced by a relatively large increase of' resistance with increase of temperature of the llame sensitive resistor St. However, if the flame sensitive resistor 33 is not heatedV by the pilot flame there is insufcient current impressed upon the coil of the solenoid valve I8 to effect actuation of the valve to open position as previously described therein.

Operation of the main burner I3 Will serve to increase the temperaturev ambientto the thermostat causing movement of the bimetal arm til toward the hot contact 42. It may be assumed for the purpose of this description that the bimetal arm 150 now movesito a neutral position engaging neither the cold contact 48 nor the hot contact 2. A circuit is then established from the valve control resistor 36, the parallel mesh of solenoid valve It with its cut-out 58 and flame sensitive element 33 all in series with the transformer secondary. In this circuit the resistor 33 and resistor 52 are disconnected as will be apparent and the current through the solenoid valve I8 is consequently reduced by removal of resistor 3S from parallel relation with the valve control resistor 33. However, when the thermostat moves as describedfrom the cold contact to neutral position the current value impressed on solenoid valve I8 is not reduced below the minimum value required to hold the valve in its open or operating position, but the circuit constants are adjusted at the lowest linevoltage which may be volts at the primary so that the normal, steady operating current is slightly above the minimum Value. The resistance of the parallel circuit in operating condition is increased suliciently by the higher resistance value of the dame sensitive element 30 when heated to permit sufiicient current to the solenoid valve t0 maintain it in the open position.

After the main burner I3 has been operating for some time the bimetal arm 4i) may move to the hot contact position due to the thermostat becoming satisfied, whereupon a circuit arrangement is established from the transformer secondary and the valve control resistor 36 in series with the three-branch parallel mesh of flame sensitive resistor 33, solenoid valve IS and cut-out 58, and constant resistor 52 with the hot contact d2. The constant resistor 52 which acts as an adjusting resistor for the solenoid valve I8 is of such a Value as to shunt sufficient current from the solenoid valve I8 tocause this valve to move to position. f

When the thermostat moves fromV the hot contact 42 tothe neutral position the solenoid vvalve I8 remains closed until the bimetal arm warpsl to close the cold contact 46. In' such event the cycleis repeated due to the flame sensitive element Vill) being heated by the pilot burner. In case the llame sensitive element 30 becomes open circuited, sufficient current flows through the cutout .58;to cause it to operate to close the valve, providing the thermostat is inl either the cold contact position or neutral position. If, however, the thermostat is in the hot contact position, the solenoid valve I8 is closed due to the shunting action of the resistor 52. V

The foregoing sequence of operation continues during the normal running condition of the burner to control a condition caused by operation of the burner. The operation of the safety device will now be described. o

If the Aflame of the pilot burner I2 becomes exclosed tinguished for anyreason when the bimetal arm viewed in Fig. 3, and a separation of the cold contact 46 from arm 40 can occur as soon as the arm 44 becomes heated by passage of current therethrough. The separation of the cold contact thus has the same effect as movement of the arm 40 to neutral position` and the valve closing timeisgenerally short. Y It will be apparent that a heater coil could be used in place of conducting current directly to the arm 44 and such variation warping from the cold'contact position, the soley noidzvalve I8 is open. IThe flame sensitive element 3l)Y will cool in the `absence of pilot burner name and its resistancewill decrease causinga corresponding decrease in current value to the solenoid valve I8 which then closes. Such closing of the solenoid valve occurs in a relatively short time Vdue to the combined actionof the flamesensitive resistor and the valve rcontrol resistor 36. As previously described, the current impressed upon the solenoid valve I8 in the neutral position of the thermostat is only slightly above the minimum value required to hold the valve in its `open or operating position. Thus, by making the valve control resistor 36 sensitive to Vcurrent changes the decrease in voltage drop across the coil'of the solenoid valve will be accelerated` whenever an increase of resistance of the valve control resistor occurs upon cooling of the flame sensitive resistor 36 and consequent increase in current through the valve control resistor. The resulting temperature increase in valve control resistor 36 causes increase in voltage drop across this resistor and the rapid decrease in voltage drop across solenoid valve I8,

ln order to make the adjustment of the circuit constants less critical and the shut-01T times of the solenoid valve comparatively short, the cold contact 46 of the thermostat is mounted on the bimetal arm 44. Thus, uponclosure of the cold contact the bimetal arm 44 will warp, after the circuit has been established, from the position shown in Fig. 2 to that shown in Fig. 3. Even if the thermostat arm 40 remains in cold contact position, the contacts separate and rejoin several times as the arm 44 heats and cools upon automatic making and breaking of the circuit. Thus, the bimetal arm 44 serves to open the cold contact 46 independently of the controlled temperature. when the cold Contact remains closed for a givenlength of time.` VThe normaloperation of the system is not affected by this action o f the arm 44 as once the solenoid valve is actuated to open position it remains open until the hot contact Yis made; Due to the provision` of the stop 4B, the thermostat"armv 40 is restrained from following movement ofthe arm 44 to the left as is Within the scope of this invention.

It will be understood that aconstant resistor could beused in place ofA the current sensitive resistor asthe valve control. then operate over a narrower range of voltages and the valve closing times would be increased. However, the system would not in such case be quite as sensitive as the preferred form to rapid voltage variations, and the advantages of reduced valve closing times and an extension of the' primary voltage range over which the system will operate are believed superior. Considerable flexibility of circuit arrangement results as the value of the respective resistances canbe varied to suit diiierent operating conditions. The single solenoid valve functions as condition and safety control in the arrangement described and it will be apparent that various changes may be made in the form .and arrangement of parts within the scope of the appended claims.

We claim:

y l. A control system for fuel burners main and pilot burners comprising in combination, electrically operable control means movable between positions for controlling the main burner, a controller in circuit with said means and responsive to changes in a condition caused by operation of the main burner, a ballast resistor exposed to the heat of the pilot burner iiame and electrically connected to said control means for varying theV current value supplied thereto upon variations in the resistivity of said resistor due to the presence or absence of said flame, and a plurality of resistance circuits adapted to be establishedalternately'by operation of said controller and including said control means and said ballast resistor, said resistance circuits having different resistance values for varying the current values to said control means, said means being movable to one position upon establishment of one said resistance circuit for impressing a predetermined current value thereon and maintained in said position upon establishment of another said circuit for impressing a different current value thereon, said resistor being adapted by said variations in resistivity thereof for cooperating with said resistance circuits to determine whether said control means is moved and maintained in said position.

2. A control system for fuel burners having main and pilot burners comprising in combination, electricallyoperable control means movable between Ypositions for controlling the main burner, alcontrol switch having a pair of contacts adapted to be opened and closed for controlling current supplytosaid means in response to changes in a condition caused by operation ofv the main burner, a ballast resistor exposed to heat `of the pilot burner flame and electrically connected to said control means for varying the current value supplied thereto upon variations in the resistivity of said yresistor due to the presenceor absence of said flame, a resistance circuit adapted to be established upon closure of said contacts and including saidcontrol means andsaid ballast resistor for adjusting thev cur- The system would having rent value to said control means and causing movement thereof to one position, and a second resistance circuit established upon opening of said contacts and. includingr said control means and said ballast resistor, said second resistance circuit having a resistance value difierentifrom the first said resistance circuit for changing the current value to said control means, the last said current value being insufilcient to. cause said movement but suicient to maintain said position, said resistor being adapted by said variations in resistivity thereof for cooperating with both said resistance circuits to determine whether saidl control means is moved' and maintained in said position.

S. A control system for fuel burners having main and pilot burners comprising in combination, electrically operable control means movable between positions for controlling themain burner, a control switch having a pair of contacts adapted to be opened and closed for controlling current supply to said control means in response to changes in a condition caused by operation of the main burner, means for rendering one of said contacts responsive to current flow acting to open said contacts when closed, a ballast resistor exposed to heat of the pilot burnerv llame and electrically connected to said control means for varying the current value supplied thereto upon variations in the resistivity of said resistor due to the presence or absence of said flame, a pair of resistance circuits adapted to be established alternately by closure and opening of said contacts and including said control means and said ballast resistor, said resistance circuits having different resistance values for adjusting the current value to said control means, said control means having suiiicient current value impressed thereon when said contacts are closed. to cause movement thereof to one position and having only suliicient current value impressed thereon when said contacts are open to maintain said one position, said resistor being adapted by said variations resistivity thereof for cooperating with both said resistance circuits to determine whether said control means is moved and main.. tained in said position.

4. A control system for fuel burners having main and pilot burners comprising in combination, electrically operable control means movable between` positions for controlling the main burner, a control switch having a pair of contacts adapted to be opened and closed for controlling current supply to said means in response to changes in a condition caused by operation of the main burner, means for rendering one of said contacts responsive'to current flow acting to open said contacts when closedfa ballast resistor connected in parallel circuit with said control means and exposed to heat of the pilot burner flame for varying the current value supplied thereto upon variations in the resistivity of said resistor due to the presence or absence of said llame, and a pair of resistances in mutual parallel relation and jointly connected in series circuit with said control means and said ballast resistor, one of said resistances being connected in said series circuit by operation of said contacts and being cooperable with the other said resistance for adjusting the current value to said control means when said contacts are closed whereby said control' means is caused to move to one of said positions upon the variation in resistivity of said ballast resistor due to the presence of said iiame.

5. A control system for fuel vburnersA having main and. pilot burnersI comprising in combination, electrically operable control means movable betweenpositions'fcr controlling the main burner,a control switchf having hot and cold contacts for controlling current supply to said means in response tozchaniges in ar condition caused by operation of the main burner, said switch Ibeing movable to a neutral position for establishing circuit conditions differentfrom those established in said hot. and cold contact positions, a ballast resistor exposed to heat ofY the pilot burner flame and electrically connected to said control means for varying the current value supplied thereto upon variations.V in` the'. resistivityY of said resistor due to the presence or absence of said flame, a resistance circuit adapted to be" established upon closure of said cold contacts and including said control means and said ballast resistor for adjusting the current value to said control means and causing movement thereof to one position if the resistivity of said resistor has varied as aforesaid due to the presence 0f a flame, meansio-r rendering said cold contacts responsive to current ilow in said circuit to cause movement thereof to said neutralr position, a second resistance circuit including said control means and said ballast resistor having a greater resistancel value than the first said circuit and established in said vneutral position for causing a reduction in the current Value to said control means to an amount suicient to maintain'said one position thereof' but insulcient to cause said movement of said control means, said ballast resistor being effective by said variations in resistivity thereof under both said circuit conditions to determine whether said control means is moved and maintained in said position, and means establishing athird resistance circuit upon closure of said hot contacts and including said control means, said ballast resistor and said second resistance circuit, said third resistance circuit having a greater resistance value than said second resistance circuit for reducing the current value to saidl control means and causing: movement thereof from said one position to initial position irrespective of the heated or unheated condition of said Iballast resistor.

6. A control system for fuel burners having main and pilot burners comprising in combination; electricallycperable control means movable between positions for controlling the main burner, a control switch having hot and cold contacts for controlling current supply to said means in response to changes in a conditionA caused by operation of the main burner, said switch being movable to a neutral position for establishing circuit conditions different from those established in said hot and cold contact positions, a ballast resistor connected in parallel circuit with said control means and exposedV to heat of the pilot burner flame and electrically connected to said control means for varying the' current value supplied thereto upon variations in the resistivity of said resistor due to the presence or absence of said name, a pair of resistances in mutual parallel relation and adapted to be jointly connected in series circuit with said control means and said ballast resistor, one of said resistances being connected in said series circuit by closure of said cold contacts and being thereby cooperable with the other said resistance for adjusting the current value to said control means to cause movement thereofto one of said positions upon the variationY in` resistivity oi said ballast resistor due tol the presence of said flameV as aforesaid, and a s l9 thirdv resistance adapted to' be connected in parallel circuit with saidfcontrol means and said ballast resistor and in series circuit with said other resistance upon closure of said hot contacts for adjusting the current value to said control means and causing movement thereof from said one position toinitial position irrespective of the heatedor unheatedV condition of said ballast resistor. Y f e 7. In a combined temperature and safety control for fuel burners, the combination with electrically operated fuel control means and a plurality of electrical circuits for said means having different resistance values, of a control switch operable for alternatively establishing said circuits and thereby varying the currentvalue to said means, said switch comprising a thermally responsive element movable in response to changes in ambient temperature caused by operation of Va burner, a second thermally responsive element movable in response to passage of electric current therethrough, switching means operable by movement of said rst element upon one ambient temperature condition to establish `one of said circuits and cause variation in the current value to said control means, said second element being moved upon said operation of said switching means to discontinue said passage of current therethrough and cause return Vof said second element to initial position to establish a second one of said circuits and another variation in the current value to .said control means; and second switching means operable by movement of said first element upon another ambient temperature condition to establish a third one of said circuits and cause a'further variation in the current value to said control means. i

8. The combination with a fuel burner Asystem comprising a main fuel burner, a, pilotfburner, electrically operable control means, a controller, and an electrical circuit including said'vcontroller and control means for controlling said main burner, of a first resistor exposed to the heat of the pilot burner llame for varying the current value supplied thereto upon variations in the resistivity of said resistor due to thepresence or absence of said flame, a pair of resistors one of which changes its resistivity upon a change in temperature of said one resistor due to the, ow of current in said circuit, means associated with said controllerY for connecting said pair of resistors in said circuit in mutual parallel relation and in series circuit with said control means and rst resistorin mutual parallel relation thereby causing sufhcient current to be impressed on said controlY means to actuate itto one controlling position when said rst resistor is heated, and means Y'associated withsaid controller and responsive to current flow in said circuit for electrically disconnecting the other one of said pair of resistors from said circuit when said control means is in said one controlling position, said first resistor and said one resistor having a combined resistance value such as to cause only suicient current to be impressed upon said control means to main` tain it in said position but insufficient to effect actuation thereof. Y

9. The combination with a fuel burner system comprising` a main fuel burner, a pilot burner, electrically operable control means, a controller, and an electrical circuit including said controller and control means for controlling said main burner, of a first resistor exposed to the heat of the pilot burner flame for varying the current value supplied thereto upon variations in the resistivity Y10 of said resistor due to the presence or absence of said flame, a pair of resistorsone of which changes its resistivity upon a change in temperature of said one resistor due to the flow of current in said'circuit, a bimetal switch arm in said controller movable in one direction when said main burner is not operating for connecting said pair of resistors in said circuit in mutual'parallel relation and in series circuit with said control means and first resistor in mutual parallel relation thereby causing sufficient current to be impressed on said control means to actuate it to a position .for supplying fuel to said main burner when saidv first resistor is heated', a second bimetal switch -arm in said controller movable in one direction in response to current ow in said circuit for electrically disconnecting the other one of said pair of resistors from said circuit, said first resistor and said one resistor having a combined resistance value such as to cause a reduction in the current value impressed upon said control means to an amount Sufficient only to maintain said position but insufficient to effect actuation thereof, and means cooperable with said first bimetal arm upon'movement thereof in another direction'forintroducing additional resistance in parallel circuit withY said control means While the current value thereto is so reduced to render said current insufficient to maintain said position and cause discontinuance of fuelfsupply to the main burner. s l e 'f 10. A control system for fuel burners having main and pilotburners, comprising an electrically operable device for controlling supply of fuel to the main burner, said fuel supply being contingent upon current of a predetermined minimum value being suppliedto said device, a source of current supply for said device and being of at least said minimum value, a ballast resistor electrically connectedfto said device and exposed to the heat of a ame at the pilot burner for varying the current value to said device according to variations in the resistance of said ballast resistor dueto the presence or absence of said flame, a resistance element in circuit with said device and ballast resistor and cooperable with said ballast resistor to cause said current value to fall below said minimum upon the variation in resistance of said ballast resistor due to the extinguishment of the f flame at the pilot burner, a controller responsive to changes in a condition caused by operation of the main burner and means controlled by said controller fory introducing additional resistance into rsaid circuit and being cooperablev with said resistance element to cause said current value to fall below said minimum irrespective of the heated condition of said ball-ast resistor.

K l1. A control system for fuel burners having main andpilot burners, comprising an electrically operable device for controlling supply of fuel to the main burner, said fuel supply being contingent upon current of a predetermined minimum l value being supplied to said device, a source of current supplyV for said device and being of at least said minimum value, a ballast resistor electrically connected to Ysaid-source in shunt with said device and exposed to the heat of a flame at the pilot burner for varying the current value to said device according to variations in the resistance of said ballast resistor due to the presence or absence of said flame, a resistance element cooperable with said ballast resistor and connected in series circuit with said resistor and device to cause said current value to fall below said minimum upon the variation in resistance of said ballast resistor due to the cxtinguishrnent of the fiame'at the .pilot burner, a-controll'er responsive to changes in a condition causedby operation of the vmain bur-ner, and means controlled iby ysaid controller for introducing additional resistance in shunt with said `device :and being cooperable withfs'ai-d resistance element to cause said Vcurrent', value to fall 'below ,saidminimum irrespective of theheated conditicn'of said ballast resistor.

l2. A control system lfor fuel 4burnersxhaving 'main and pilot burners, comprising electrically operable means ,movable .between positions for controlling the fuel supplyto ,the `main burner, a ballast resistor having -a preselected temperai ture coefficient of yresistance :locatedto be heated by the pilot burner `and electrically connected'to said means, a control switchfor said means `including a plurality of pairs .of contacts adapted to be alternatively closed inresponse 'to :changes ina condition caused by operation of the ymain burner, .separate circuits for .said means established respectively by closure of said contact pairs and having different resistance values, one-ofsaid circuits having a resistance value iin the heated condition `of said -ballas't resistor such that Athe current vvalue to said means is Isu'flicient lto cause movement thereof to l-fuel supply position and another said circuit having a `resistance value such that the ycurrent-value is insufficient to maintain said means in said fuel supply position, and acircuit for -said lmeans established linltl'ie `open position of all .of said Contact pairs Yand having a resistance value `intermediate those of saidseparate circuits for rendering the current value ineffectiveto ycause said means to change Athe'position to which it has Vbeenn'loved Iby establishment of said one or anotherof said'separate-circuits, `said `ballast resistor being eifective `upon Y extinguishment of said flame for increasing'the resistance value Vof the last-saidvcircuit and thereby reducing the current value 'to saidmeans sufficiently to cause movement thereof from saidfuel supply position.

13. A control system for .fuel burners having main and 'pilot burners, comprising electrically operable means movable 'between Apositions for controlling the supply of fuel to the main burner, a ballastresistor electrically `connected to said means and responsive to the presence or absence of a flame .at the `pilot burner for varying the current value supplied to said means, a thermostatic control'svvitch for said.m'eans including'two pairs of contacts adapted to be alternatively closed in response to changes in a temperature condition causedby operationofthe mainburner, a circuit for said means established upon closure of one of said contact pairs on Ya drop in .said temperature andhavinga resistance value such that the current value to said meansis sufficient to cause movement thereof to fuel supply position, a second circuit for said means established in the open ,position of said two `contact `pairs on a rise insaid temperature and having aresistance value :greater than that Vof the Afirst said ,circuit to reduce the current value .to said means but insuiiiciently 'to `.cause movement from Asaid -fuel supply position, a'third circuit for said Ymeans established `upon closure of `the other of said contactpairs on a A`further rise in said' temperature yand having sufficiently greater resistance than said second circuit to render the current value to `said means insufficient to maintain said means in fuel supply position in the heated vcondition of said ballast resistor, and thermally responsive means operable to cause opening of said one contact pair to establish said second circuit irrespective of said temperature condition.

14. YIna control system for fuel burners, electrically operable'fuelfcontrol-means arranged for fuel supply to -a burner when said means is vplaced in energized condition and for preventing .said fuel :supply when placed in effectively deenergized condition, a control switch ,for said means including a'plurality of contact pairs 'adapted to be nalternatively closed 1in response to lchanges-in a condition caused 'by'operation :of the burner, said `svi/itch .having an :open position vof all of said contact pairsyacircuit for said fuel controlineans initially yestablished in the -zopen `position fof said switch, meanslresponsiveto closure of oneofzsaid Contact pairs .for reducing the :total 'resistance Value of lsaid circuit sufliciently to cause said fuel controlrneansto be'placed in said energized condition, Iand means responsive to closure .of another .one of said Contact pairs for increasing vthe;tota`l resistance Vvalue ofzsaid circuit sufficiently to .cause said fuel control 'means lto Ibe rplaced in said effectively deenergized condition, said circuit as Ainitially established having a vresistance value suchthatsa'id control means ismaintained in the condition in :Whichit has 'been placed 'by a preceding.closureofeither said one or the other ofY said contact pairs.

l5. In a control system for fuel -burners,elec trically operable fuel control means arranged for fuel supply to a'burner whensaidmeans is placed in energized condition and for preventing said fuel 'supply when placed in effectively ydeenergized condition, a control switch vfor said Ameans including a plurality of contact pairs adapted to be alternatively closed in response yto changes in-a condition caused byoperation ofthe burner, said switch having -an open position of all of Vsaid contactpairs, acircuit for said fuel control means initially established in the open position of said switch and including ya resistor, means responsive to Vclosure of one of saidcontact pairs for intro-- ducing resistance means in parallel circuit with said resistor for reducing lthe total resistance value of said circuit sufficiently to cause said fuel control means to beplaced in saidenergized condition, and means responsive to closure of another oneof said contact pairs for introducing another resistance means in parallel circuit withsaid fuel control means for increasing the Atotal resistance value of said circuit sufciently to cause 'said fuel control :means to be placed in said 'eifectively deenergized condition, said circuit as initially established having a resistance value without either of said resistance means such that said control means is maintained in the condition in which it hasbeen placed by apreceding closure of either saidone or the other ofsaid-contact pairs.

SAMUEL G. ESKIN. `CI-llAltLES 4K. STROBEL. 

